Reduce Your PCB Costs With Blind Vias, Buried Vias & Microvias

Although HDI PCB fabrication can be expensive, there are several situations where blind vias, buried vias, and microvias offer a cost-effective solution.

High Density Interconnect (HDI) circuit board designs have a higher wiring and pad density than conventional PCBs, along with smaller trace widths and spaces. They require advanced PCB technologies such as blind vias, buried vias, and microvias. HDI PCBs are usually more costly than conventional PCBs due to the complex build-up process involved in fabrication.

Although HDI PCB fabrication can be expensive, there are several situations where blind vias, buried vias, and microvias can offer a cost-effective solution. Here are just a few scenarios where advanced PCB technologies may actually reduce the overall manufacturing cost.

Scenario 1: Reduce layer count by widening the BGA breakout channel with a blind via
If you're using through-hole vias for a design and the trace escape for a ball grid array (BGA) isn't going smoothly, consider widening the breakout channels on the bottom and inner layers with blind vias.

Consider the illustration below. The board on the top had three rows of through-hole vias, letting only four traces out between the 1st and 3rd rows.

By comparison, in the case of the board on the bottom, the via in the middle has been replaced with a blind via. The same channel now permits six traces, which equates to a 50% increase. This technique may allow you to reduce the layer count of your PCB and -- in turn -- reduce the cost.

Scenario 2: Eliminate electrical layers by replacing through-hole vias with microvias
Microvias have the smallest pad size (as little as 0.008"), which helps maximize routing channel width. As in the previous scenario, replacing through-hole vias with microvias can ease BGA breakout and reduce the PCB layer count. When an inner signal layer is eliminated, its reference plane can sometimes be removed too, thus eliminating two layers.

The illustration below demonstrates one of the techniques that can be used to maximize breakout between microvias for a 1.0mm pitch BGA:

If placing the microvias for maximum breakout, the gain will be 67%. If placing the microvias at the same grid as the through-hole vias, the breakout gain would be 33%.

Scenario 3: Use blind and buried vias to reduce the PCB aspect ratio
Often, a PCB will have BGA components with several different pitches. For example, there may be a 1.27mm pitch BGA and an 0.8mm pitch BGA on a 4.0mm (160mil)-thick PCB. The minimum via hole is not only determined by drill size, but also the aspect ratio, which is the thickness of the PCB divided by the diameter size of the drilled hole:

There is usually an additional manufacturing cost for aspect ratios higher than 10 or 12. (Note that the finished hole size is ~2mil less than the drilled hole size; this due to the width of the barrel plating.)

If through-hole vias are still required, even after blind/micro/buried vias are used, it may be a challenge keep the aspect ratio low. An aspect ratio of 12 on a 4mm board would mean that the minimum finished through-hole via size is 0.3mm (12mil), which corresponds to a drill hole size of 0.36mm (14mil) and pad size of 0.56mm (22mil). This would be fine for the 1.27mm pitch BGA, but would raise concerns for the 0.8mm pitch BGA, since the via pad-to-BGA pad clearance would only be 0.09 (3.4mil) with 0.4mm (15.7mil) BGA pads.

To meet the aspect ratio requirements, the PCB thickness would have to be reduced as follows:

If you reduce the layer count by replacing through-hole vias with blind vias and microvias, you can achieve a smaller aspect ratio for your PCB, and avoid any cost penalties due to using blind/microvias.

These are just three scenarios where advanced PCB technology may be cost-effective for your design. Look for many more unique situations soon, as PCB fabrication equipment and materials continue to progress.

— Pi Zhang is a Senior Design Engineer at Nuvation Engineering, a design firm specializing in new product development.

Good summary! Just little complement. In my experience, for 1.0mm or smaller pitch BGA pin fanout, the maximum amount of reduction of layers due to blind-via or micro-via actually depends on the order of via though the increase of line space is more. For one order blind-via based on two times-laminate or one order micro-via from HDI, the amount of layer will reduces only only signal layer or a couple layers.

So,the cost-saving only from the reduction of layers usually is not enough to let HDI solution become cost-effictive. To effect of aspect ratio, it leverages as the PCB thickness is more than 3mm or more than 24 layers than 24 with 1.0 or less pitch BGA application.

For other application scenario, as well as double-side mirror layout the BGA such as memory devices to maximizing the space of layout.

we had to get a sensor module sealed from external humidity. As it is, tracks on the top layer do not work too well with an O-ring seal :(

The solution were blind vias - water tight without the additional costs for plugged vias.

Wish we had started with blind vias sooner (4-layer PCB) as it became clear very soon that they give you a lot of space to route on 2 layers not affected. If there is no requirement for full-blown supply/reference layers you can virtually have 2 independent 2-layer boards to route,

Thank you for your response. I agree completely with everything you stated in your response. The article is well written (other than the title), and addresses aspect ratio issues which are a significant issue with high density boards using microvias. The issues surrounding aspect ratio are not adequately addressed or explained in enough detail for the average PCB designer. Thank you for taking the time to respond.

Thanks for the comment! Like the article describes, using blind and buried vias can reduce your layer count and aspect ratio (just to name a few of the benefits), which can have a big cost impact on high layer count TH PCB. For the PCBs with 16- layers, HDI technology is probably not cost-effective, but when PCB layer count reaches 20+ I find that a) using small TH vias is limited by board aspect ratio and b) 1.00mm or bigger pitch BGAs are necessary for proper breakout. This leads to a larger PCB size, and Blind/Micro/Buried vias can play a role in reducing layer count and PCB size.

The cost model for HDI PCB is not as straightforward as TH PCB, as it is closely tied to HDI layer stack design and varies between boards having same layer count but different HDI layer stack. While HDI technology may be the most cost-effective option for a product, you're absolutely correct that the HDI technology itself is more expensive. I suppose a more accurate title would be "Cost-effective Options For High Layer Count TH PCBs That Use Blind, Buried and Micro Vias".